The present invention relates generally to papermaking fiber processing and more particularly to a method and apparatus useful for cleaning secondary pulp by way of a multistage forward cleaner system with an integrated flotation cell which cooperates with the forward cleaners to boost efficiency of the system.
Processing of papermaking fibers to remove contaminants is well known in the art, including the use of forward cleaners and flotation cells. Such technology is used, for example, to treat secondary (recycle) fiber sources for re-use in paper products such as towel and tissue, paperboard, coated writing and printing papers and so forth. Following is a brief synopsis of some patents of general interest.
According to U.S. Pat. No. 4,272,315 to Espenmiller waste paper containing materials, e.g., commercial xe2x80x9cwaste paperxe2x80x9d, are treated for recovery of reusable paper therefrom by slushing in a pulper from which two fractions are continuously extractedxe2x80x94a first fraction through small holes, e.g. {fraction (3/16)} inch in diameter, and a second fraction through substantially larger holes, e.g., 1 inch in diameter. The second fraction is screened, preferably after a centrifugal cleaning operation, in a screen having small perforations sized to accept only substantially defibered paper, and the accepts flow is mixed directly with the first extracted fraction. The reject flow from this screen is conducted, with or without an intermediate deflaking operation, to a tailing screen from which the accepts are recycled to the pulper and the rejects are eliminated from the system. Advantages of this method and system include the continuous elimination of plastic and other floating trash from the pulper, a high degree of essentially complete defibering in the pulper, and minimal recycling of adequately defibered stock.
U.S. Pat. No. 4,983,258 to Maxham discloses a process for the production of papermaking fiber or pulp from waste solids emanating from pulp and paper mills, particularly waste solids in process water streams containing fibrous solids that cannot be directly recycled by paper mill xe2x80x9csaveallxe2x80x9d devices, from pulp and paper mill process water streams conveyed by the sewerage system to wastewater treatment plant facilities, and from xe2x80x9csludgexe2x80x9d emanating from the underflow of a primary clarifier or sedimentation basin at pulp and paper mill wastewater treatment facilities either before or after the xe2x80x9csludgexe2x80x9d is thickened and dewatered. The said process comprises a defibering stage to release individual fibers from bundles, a screening stage to separate long fiber and debris from short fiber and clay, a centrifugal cleaning stage to separate debris from the long fiber, a bleaching stage to increase the brightness of the fiber, a dewatering stage to remove excess water from the pulp, a sedimentation stage to separate the short fiber-clay-debris from the defibering effluent which is substantially recycled, and a biological treatment process to remove dissolved organic materials from the excess water generated which can be either discharged from the process or recycled as process water.
U.S. Pat. No. 5,240,621 to Elonen et al. discloses a method of separating an aqueous solids containing suspension which includes (a) subjecting a first solids containing suspension to centrifugal forces so as to separate the suspension into a first gas containing flow, a second gas-free flow and a third flow; (b) feeding the third flow into a flotation cell having a bottom; (c) introducing air at the bottom of the flotation cell into the third flow for separating from the third flow a fourth partial flow; (d) withdrawing the air containing third flow after the separation of the fourth partial flow from the flotation cell; and (e) subjecting the third flow to the centrifugal forces of step (a). An apparatus for the separation of gas and lightweight material from a gas and lightweight material containing aqueous solids suspension is also described and includes a centrifugal pump for separating the gas and lightweight material from the solids suspension with a suspension inlet and an outlet for the lightweight material; a flotation cell for separating the lightweight material from a solids suspension; and a circulation loop connecting the outlet of the centrifugal pump, the flotation cell and the suspension inlet of the pump.
In U.S. Pat. No. 5,693,222 to Galvan et al. a dissolved gas flotation tank system is disclosed which is configured to provide educted gas or air into recirculated effluent fluid from the tank which includes a pump system which increases the dissolution rate of gas into the effluent fluid thereby eliminating the need for retention tanks and related equipment which adds to high equipment costs. The dissolved gas flotation tank system also provides a pre-contact chamber for assuring immediate and intimate contact between the suspended solids in an influent feed stream and the recirculated effluent fluid in which gas is dissolved, as well as flocculant when used, to produce a better agglomerate structure for improved flotation and separation. The dissolved gas flotation tank also provides an improved means of removing and processing float from the tank, and employs a dewatering system enhanced by the addition of chemicals or flocculants into the float removal system.
The disclosures of the foregoing patents are hereby incorporated for reference.
While flotation and separation technologies are fairly advanced, there is an ongoing need to increase overall fiber-cleaning system performance and to reduce the amount of waste and capital investment in the plant.
The present invention provides a hybrid system for processing papermaking fibers and includes a multistage array of forward cleaners coupled with a flotation cell which increases overall efficiency of the system. In a typical embodiment, a first rejects aqueous stream from a first stage bank of centrifugal cleaners is treated in a flotation cell before being fed to a second stage bank of centrifugal cleaners.
One advantage of feeding the second accepts stream forward is that it does not have to be returned to the first bank of cleaners for re-cleaning. This reduces the size of the first bank of cleaners or allows an existing installation to operate at a lower consistency. (The cleaners operate more efficiently at a low consistency of 0.5% than at 0.8 or 1%). Another advantage is that the flotation cell operates at greater than 60% efficiency on removing hydrophobic contaminants from the first cleaner rejects, while another cleaner stage removes less than 50% of the hydrophobic contaminants. As a result a large quantity of hydrophobic contaminants are removed in the flotation stage, which makes the remaining cleaner stages work more efficiently with less good fiber loss.
Investigation showed that the number of hydrophobic contaminants in the second cleaner accepts after the flotation stage was lower than the number of hydrophobic contaminants in the first cleaner accepts. Without the flotation stage the number of hydrophobic contaminants in the second accepts is much higher than the first accepts, so that the second accepts have to be returned to the first bank of cleaners for more cleaning.
As will be appreciated from the discussion which follows, the size and cost of a flotation stage for treating secondary fiber can be reduced by up to 75% if it is installed in centrifugal cleaner system as compared to a full scale treatment of the stock by flotation. The centrifugal cleaner system modeling indicates a 34% reduction in ink speck area of total centrifugal cleaner system accepts by removing ink specks from the first stage rejects with 80% efficiency in a flotation stagc and then feeding the flotation accepts forward after centrifugal cleaning of the second stage. (24% reduction if second stage rejects are treated in a similar manner). The ability to feed the centrifugal cleaner rejects forward (after the flotation stage and additional centrifugal cleaning in the next stage) reduces the stock consistency in the first stage, thereby improving the efficiency of He first stage. The capacity of the system is also increased by feeding the second stage centrifugal cleaner accepts forward. The other centrifugal cleaner ages can also be operated more efficiently since more than 50% of the ink in the first stage centrifugal cleaner rejects has been removed in the flotation stage. When the centrifugal cleaner accepts are thickened in a press, a large amount of ink ends up in the pressate. This ink can also be removed by using the ink-laden pressate as dilution water for the centrifugal cleaner rejects going to the flotation stage.
A conventional centrifugal cleaner system (as shown in FIG. 1) normally consists of several stages, whereby the rejects of each centrifugal cleaner stage are diluted for cleaning in the next stage and the centrifugal cleaner accepts are fed backwards to the feed of the previous stage. The ink speck removal efficiency of the centrifugal cleaner is usually much less than 50% on toner inks in office waste paper. As a result the total centrifugal cleaner system ink speck removal efficiency can drop to 30% or less on a furnish containing a large proportion of office waste.
By sending the first or second stage centrifugal cleaner rejects to a flotation stage (as shown in FIG. 2) it is possible to remove a much higher percentage of the ink specks in office waste. (It was possible to obtain 80% removal of ink specks during a pilot plant trial with a flotation cell operated on second stage centrifugal cleaner rejects.) If the accepts of the flotation cell are cleaned in the next centrifugal cleaner stage, the centrifugal cleaner accepts from that stage can then be fed forward to the thickener. Sending centrifugal cleaner accepts forward reduces the load and improves the efficiency of the previous centrifugal cleaner stage.
The present invention is particularly useful in connection with removing stickies from the recycle fiber product stream; likewise, it is believed pitch removal is enhanced. Stickies are generally a diverse mixture of polymeric organic materials which can stick on wires, felts or other parts of paper machines, or show on the sheet as xe2x80x9cdirt spotsxe2x80x9d. The sources of stickies may be pressure-sensitive adhesives, hot melts, waxes, latexes, binders for coatings, wet strength resins, or any of a multitude of additives that might be contained in recycled paper. The term xe2x80x9cpitchxe2x80x9d normally refers to deposits composed of organic compounds which are derived form natural wood extractives, their salts, coating binders, sizing agents, and defoaming chemicals existing in the pulp. Although there are some discrete characteristics, there are common characteristics between stickies and pitch, such as hydrophobicity, low surface energy, deformability, tackiness, and the potential to cause problems with deposition, quality, and efficiency in the process. Indeed, it is possible with the present invention to reduce stickies by 50%, 80% or even more by employing a flotation cell in a multistage forward cleaner system as hereinafter described in detail.
The rejects from the flotation stage are so full of ink and ash that they can be rejected without any further treatment.
There is provided in one aspect of the present invention, a method of processing papermaking fibers with a multistage array of forward cleaners including a plurality of centrifugal cleaners configured to generate accepts streams and rejects streams which concentrate heavy waste, the method including (a) feeding a first aqueous feed stream including papermaking fibers to a first stage bank of centrifugal cleaners of the multistage array; (b) generating a first accepts aqueous stream and a first rejects aqueous stream in the first stage bank of centrifugal cleaners, the first aqueous rejects stream being enriched in heavy waste with respect to said first aqueous feed stream; (c) supplying the first rejects aqueous stream to a flotation stage; (d) treating the first rejects aqueous stream in the flotation stage to remove hydrophobic waste from the first aqueous rejects stream and produce an intermediate aqueous purified feed stream; and (e) feeding the aqueous purified intermediate feed stream to a second stage bank of centrifugal cleaners of the multistage array, the second centrifugal cleaner being configured to generate a second accepts aqueous stream, wherein the second rejects aqueous stream is enriched in heavy waste with respect to said aqueous purified intermediate feed stream. The method may further include feeding the first accepts aqueous stream and said second accepts aqueous stream to another cleaning device or a thickening device. Suitable additional cleaning devices include screening devices, reverse cleaners and the like. In a preferred embodiment, the first aqueous feed stream comprises a preliminary accepts stream generated by way of a preliminary bank of centrifugal cleaners dividing a preliminary feed stream into a preliminary accepts stream and a preliminary rejects stream. A preferred method may include feeding the preliminary rejects stream to the flotation stage and treating the preliminary rejects stream along with the first rejects aqueous stream to remove hydrophobic waste therefrom whereby the aqueous purified intermediate stream includes treated components from both the preliminary rejects stream and the first rejects aqueous stream.
In other preferred embodiments, the process may include feeding the second rejects aqueous stream to a third centrifugal cleaner operative to generate a third accepts aqueous stream and a third rejects aqueous stream.
Preferably, the multistage array of forward cleaners comprises at least 3 banks of centrifugal cleaners, and still more preferably, the multistage array of forward cleaners comprises at least 5 banks of centrifugal cleaners. The first aqueous feed stream generally has a consistency of from about 0.3% to about 0.9%, whereas the first aqueous stream more typically has a consistency of from about 0.4% to about 0.7%. The hydrophobic waste removed from the first aqueous stream by the flotation stage often includes an ink and stickies composition, toner ink compositions being typical in office waste and stickies compositions frequently being obtained from pressure sensitive adhesives in office waste.
In another aspect of the invention there is provided a hybrid apparatus for processing papermaking fibers with a multistage array of forward cleaners including (a) a first bank of centrifugal cleaners configured to generate a first accepts stream and a first rejects stream upon operating on a first aqueous feed stream, the first rejects stream being enriched with respect to heavy hydrophobic contaminants with respect to the first aqueous feed stream; (b) a flotation cell connected to the first bank of centrifugal cleaners so as to receive the first rejects stream and adapted to remove hydrophobic contaminants such as ink, stickies and the like from the first rejects stream, the flotation cell being constructed and arranged so as to generate a flotation rejects stream and a flotation accepts stream which is purified with respect to hydrophobic contaminants in said first rejects stream; and (c) a second bank of centrifugal cleaners coupled to the flotation cell so as to receive the flotation accepts stream as a second feed stream, the second bank of centrifugal cleaners being likewise configured to generate an accepts stream hereinafter referred to as a second accepts stream and a second rejects stream respectively. In a preferred embodiment, a preliminary bank of centrifugal cleaners is provided upstream of the first bank of centrifugal cleaners and coupled thereto whereby the accepts stream of the preliminary bank of centrifugal cleaners is fed to the first bank of centrifugal cleaners. The banks of centrifugal cleaners are typically hydrocyclone type cleaners.
Unless otherwise indicated, terminology appearing herein is given its ordinary meaning; %, percent or the like refers, for example, to weight percent and xe2x80x9cconsistencyxe2x80x9d refers to weight percent fiber or solids as that term is used in papermaking.